Vehicle systems may include an engine with an exhaust gas treatment system coupled in its exhaust passage in order to control regulated emissions. In some examples, the exhaust gas treatment system may include a selective catalytic reduction (SCR) system in which an exhaust fluid, such as urea or ammonia, is added to the exhaust stream upstream of a catalyst such that NOx may be reduced by the catalyst. In such an example, the exhaust fluid may be delivered to the exhaust passage via an injector fluidically coupled to a pump by an exhaust fluid passage. An exhaust fluid sensor may be disposed in the exhaust fluid passage in order to determine the quality of the exhaust fluid for emission compliance, for example. Such a sensor may degrade over time, however, resulting in reduced reliability of the sensor output.
The inventors herein have recognized the above issue and have devised an approach to at least partly address it. In one example, a method includes indicating degradation of an exhaust fluid sensor positioned upstream of an exhaust injector based on a first reading when exhaust fluid is present at the sensor and a second reading after exhaust fluid is evacuated away from the sensor.
By obtaining a first reading and a second reading under different conditions, exhaust fluid sensor degradation may be diagnosed. For example, under the two different conditions, a different exhaust fluid reading is expected. Because exhaust fluid is present at the sensor when the first reading is obtained, such as when the engine is in operation, the first reading may have a higher value than the second reading which is obtained when exhaust fluid is not present at the sensor, such as after the engine is shutdown. Thus, degradation of the exhaust fluid sensor may be diagnosed based on the first exhaust fluid reading and the second exhaust fluid reading, and a vehicle operator may be notified of the degraded sensor and/or a diagnostic code may be set.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.